56 RESPONSE IN THE LIVING AND NON-TIViae 
a common effect of magnetisation is to produce an 
elongation of an iron rod. But Bidwell finds that as 
the magnetising force is pushed to an extreme, at a 
certain point elongation ceases and is succeeded, with 
further increase of magnetising force, by an actual con- 
traction. Again a photographic plate,.when exposed 
continuously to light, gives at first a negative image. 
Still longer exposure produces a positive. Then again 
we have anegative. There is thus produced a series of 
recurrent reversals. In photographic prints of flashes 
of lightning, two kinds of images are observed, one, the 
positive—when the lightning discharge is moderately 
intense—and the other, negative, the so-called ‘ dark 
lightning "—due to the reversal action of an intensely 
strong discharge. 
In studying the changes of conductivity produced in 
metallic particles by the stimulus of Hertzian radiation, 
I have often noticed that whereas feeble radiation pro- 
duces one effect, strong radiation produces the opposite. 
Again, under the continuous action of electric radiation, 
I have frequently found recurrent reversals.’ 
Diminution of response under strong stimulus traced 
to fatigue.—But there are instances in plant response 
where the diminution effect can be definitely traced 
to fatigue. The records of these cases are extremely 
suggestive as to the manner in which the diminv- 
tion is brought about. The accompanying figures 
(fig. 34) give records of responses to increasing stimulus. 
They were made with specimens of cauliflower-stalks, one 
of which (a) showed little fatigue, while in the other (4) 
' See ‘On Electric Touch,’ Proc. Roy. Soc. Aug. 1900. 
